fix: use Python datetime for created_at in worker

Consistent with model default which uses datetime.now(UTC) rather than
SQL NOW() for created_at field.

backend/worker.py

@@ -5,6 +5,7 @@ import json
 import logging
 import random
 import time
+from datetime import UTC, datetime
 
 import asyncpg
 from pgqueuer import Job, QueueManager, SchedulerManager
@@ -116,13 +117,14 @@ async def process_bitcoin_price_job(db_pool: asyncpg.Pool) -> None:
                 """
                 INSERT INTO price_history
                     (source, pair, price, timestamp, created_at)
-                VALUES ($1, $2, $3, $4, NOW())
+                VALUES ($1, $2, $3, $4, $5)
                 ON CONFLICT (source, pair, timestamp) DO NOTHING
                 """,
                 SOURCE_BITFINEX,
                 PAIR_BTC_EUR,
                 price,
                 timestamp,
+                datetime.now(UTC),
             )
 
         logger.info(f"Fetched BTC/EUR price: €{price:.2f}")